In a hydraulic excavator such as a construction machine, a bucket mounted on the tip end of arm is swung by means of a hydraulic cylinder in excavating operation. The joint mounted between the bucket and arm is composed of a sliding bearing device, which comprises a shaft and a bearing. Because the bearing device like this is subjected to a high surface pressure, a wear-resistant bearing is used and the sliding surface of the bearing is applied with a highly viscous lubricating oil, grease or wax.
The bearing of this kind is made of an iron-copper-carbon-based sintered alloy which is impregnated with a lubricating oil of high kinematic viscosity, in place of those made of cast alloy and formed by machine work or those in which the sliding surfaces are embedded with the dispersed particles of graphite. In order to improve both the mechanical strength and wear resistance, the above-mentioned oil-impregnated sintered bearing is made of a matrix of iron-carbon-based alloy containing martensitic structure and about 20% by mass of copper phases are dispersed in the alloy structure.
The bearings are mechanically hard because they are processed through heat treatment and the sizes of them are relatively large, so that the bearings are generally manufactured by machine cutting, which is finished by grinding of their inner peripheral surfaces.
The conventional oil-impregnated sintered sliding bearings like the above features are suitably used under large loads, because they are made of quenched iron alloy containing dispersed copper phases in the alloy matrix and they are impregnated with lubricating oils. However, they are produced through the finishing work of machining and grinding, which finishing work is not simple. It is, therefore, demanded that such production processes is simplified and, in addition, the obtained bearings have the performance that is equivalent to the conventional bearings.